Across species and populations, encounters between neighboring social groups take a variety of forms. In particular, intergroup encounters (IGEs) may or may not be aggressive and may include the participation of males and/or females. The proximate causes of aggressive participation by each sex, particularly among primates, is generally thought to be the availability of mates and food. However, existing hypotheses of resource defense have rarely been explicitly tested through identification of the proximate causes of male and female aggression. In this dissertation, I sought to test the existing hypotheses by determining whether female food defense, male food defense, and male mate defense occur in grey-cheeked mangabeys (Lophocebus albigena) and redtail monkeys (Cercopithecus ascanius). With a team of field assistants, I observed six mangabey groups for 15 months and four redtail groups for 12 months at the Ngogo site in Kibale National Park, Uganda. We observed naturally-occurring IGEs in both species, simulated IGEs among mangabey groups using playback experiments, and measured the availability of food resources in botanical plots. I evaluated multiple aspects of intergroup relations, including initiation of encounters, occurrence of intense aggression, encounter outcomes, overall encounter rates, and the effect of neighbors' long-distance calls on group movements. For each IGE aspect, I determined the effects of group size and resource value. I found strong evidence for female food defense by redtails and male food defense by both species, weak evidence of male mate defense by redtails and no evidence for mangabeys, and no evidence of female food defense by mangabeys. In addition, the specific conditions under which food defense is expected to occur in primates were appropriate for male mangabeys, but not for female or male redtails. This pattern of results indicates that existing hypotheses cannot accurately predict which populations will exhibit food or mate defense by males or females, or the specific social and ecological conditions that elicit defense. I also found that mangabeys exhibited two types of IGE: whole group encounters, where the majority of two groups were in visual contact, and subgroup encounters, where one or a few individuals left their group and interacted aggressively with a neighboring group. Whereas whole group IGEs appeared to function as defense of specific feeding sites, subgroup IGEs did not; instead, they appeared to be a means of defending the core of the home range. Compared to earlier studies, the mangabey groups in this study exhibited higher encounter rates and more pronounced aggression. Higher group densities and more intense feeding competition have given rise to a dramatically different pattern of mangabey intergroup relations. This study demonstrates the importance of considering multiple IGE aspects, hypotheses, and food characteristics when evaluating the role of intergroup relations in the lives of social animals.